In a backlight type display apparatus (a liquid crystal display apparatus) illuminating a display panel (e.g., a liquid crystal display module) from its backside, a flat or plane light source unit (or a backlight unit) is disposed on the backside of the display panel. The plane light source unit comprises, for example, a tubular light source such as fluorescent tube (cold cathode tube) disposed adjacent to a lateral side of a light guide, the light guide for guiding a light from the tubular light source to a display panel, and a reflector disposed opposite to the display panel side of the light guide. In such a plane light source unit, since a light from a fluorescent tube is reflected by the reflector and guided by a light guide, a diffusing or scattering film is usually disposed between the tubular light source and the display panel for uniformly illuminating the display panel from behind. As the diffusing film, a transparent and highly heat-resistant polycarbonate film or a polyester film each comprising dispersed resin microfine particles (resin beads) or light-transmissive inorganic microfine particles is employed. However, even when such a diffusing film is used, it is inevitable due to its isotropy of light diffusing that the luminance in a certain direction (the axis-direction of the fluorescent tube) is unduly lowered. As a result, it is unable to illuminate the display panel uniformly with the high luminance.
Therefore, in Japanese Patent Application Laid-Open No. 231315/1999 (JP-11-231315A), 84357/1999 (JP-11-84357A) and 84376/1999 (JP-11-84376A), an optical element such as a prismatic lens is interposed between the diffusing film (diffuser) and the liquid crystal layer to thereby refract the diffused light so that the light will be incident perpendicularly on the liquid crystal display surface, thus upholding the luminance.
More specifically, Japanese Patent Application Laid-Open No. 231315/1999 (JP-11-231315A) discloses a plane light source unit comprising a linear light source such as a fluorescent tube, a light guide on which a light from the linear light source is incident from the lateral side, a diffusion reflector disposed adjacently to the bottom surface of the light guide, a reflecting mean composed of concavity/convex formed on the bottom surface of the light guide (e.g., a prismatic system).
Japanese Patent Application Laid-Open No. 84357/1999 (JP-11-84357A) discloses a liquid crystal display apparatus comprising a backlight unit, a liquid crystal display panel disposed on the unit, a first lens sheet interposed between the backlight unit and the liquid crystal display panel, and a change or switching means for changing the first lens sheet to a second lens sheet. The literature also describes a technology that a light from a fluorescent tube is guided from the lateral side of the light guide to an emerging surface, a light from the emerging surface is focused by a lens sheet in which a plurality of triangle-shaped sectional prisms are formed in parallel with each other, thereby a display panel is illuminated.
Such a device or unit enables to focus the diffused light with use of a prism for illuminating the display panel with the high luminance. However, an uneven emission (luminance) distribution in the direction perpendicular relative to the longitudinal direction of the fluorescent tube is inevitable, causing a streak pattern to appear, although the emission distribution in the longitudinal direction of the fluorescent tube is uniform. Therefore, it is difficult to illuminate the display panel uniformly.
Japanese Patent Application Laid-Open No. 84376/1999 (JP-11-84376A) discloses, as a unit for illuminating a transmittable liquid crystal display panel with uniform luminance, a backlight unit comprising a light guide for guiding a projected light to the display panel, a fluorescent lamp disposed in proximity to one side of the light guide, a reflector for reflecting a light from the fluorescent lamp toward a front direction (a direction of a display panel), a diffusion plate (diffuser) for diffusing a emerge light from an emerging surface of the light guide to be uniformed, which is disposed on the front side of the light guide, and a prism sheet for gathering a light from the diffusion plate. The literature describes an example of a unit comprising a pair of prism sheets disposed oppositely with aligning the extended direction of the prisms toward a crossing direction each other, and diffusion plates disposed on both sides of the prism sheets.
Since a plurality of prism sheets and a plurality of diffusers are required for such a backlight unit, its structure is complicated and its luminance is lowered. Moreover, even when the above backlight unit is employed, its luminance distribution is not still uniform. Thus, although an emission distribution (luminance distribution) in the longitudinal direction (X-axis direction) of the fluorescent tube (cold cathode tube) is relatively uniform, the emission distribution (luminance distribution) in the Y-axis direction perpendicular to the X-axis direction of the fluorescent tube has a streak-like directionality (linear dark areas) and is not still uniform.
For example, as a plane or flat display apparatus of which the image display area has a flat surface (a flat type display apparatus), an apparatus as illustrated in FIG. 8 is known. The apparatus comprises a flat display unit (e.g., a transmittable liquid crystal display unit) 45 and a flat light source unit adapted to illuminate the display unit from its back side. The plane or flat light source unit comprises at least one fluorescent discharge tube (cold cathode tube) 41, and a reflector 42 is disposed on the back side of the fluorescent discharged tube 41 for reflecting a light, a diffuser 43 is interposed between the fluorescent discharged tube 41 and a display unit 45 for diffusing the light to uniformly illuminate the display unit 45, and a prism sheet 44 is laminated on the unit side of the diffuser 43. The flat display unit 45, in the case of a liquid crystal display unit, comprises a first polarizing film 46a, a first glass substrate 47a, a first electrode 48a on the glass substrate, a first alignment layer 49a laminated on the electrode, a liquid crystal layer 50, a second alignment layer 49b, a second electrode 48b, a color filter 51, a second glass substrate 47b, and a second polarizing film 46b as successively built up (laminated) in that order. In such a display apparatus, the display unit can be directly illuminated from the back side by the built-in fluorescent tube (cold cathode tube) 41.
Moreover, an apparatus comprises a backlight unit having such a light guide as illustrated in FIG. 9 as the backlight system of the flat display apparatus of FIG. 8 has been known. This backlight unit has a fluorescent tube (cold cathode tube) 51 and a reflector member 55 disposed in parallel with the fluorescent tube, with a light guide 54 having a diffuser 53 at top and a reflector 52 at bottom being disposed in the direction of light emission from the fluorescent tube. Incidentally, the thickness of the light guide 54 at the fluorescent tube side is larger than that of the other side, so that the light from the fluorescent tube 51 can be reflected in a forward direction. The light emerged from the emerging surface of the light guide is diffused by the diffuser 53 and incident on the flat display apparatus (not shown) constructed (laminated) on the diffuser.
When such a backlight unit is used, a display panel can be illuminated by focusing a diffused light with use of a prism sheet, and in contrast to the backlight unit or component of FIG. 8, the emission distribution may appear uniform over the surface but a detailed observation of the emission distribution reveals that the distribution is still not as uniform as desired. Thus, as shown in FIGS. 10 and 11, the emission distribution (luminance distribution) in the longitudinal (axial) direction (X-direction) of the fluorescent tube (cold cathode tube) 51 is relatively uniform as it is the case in the apparatus or device of FIG. 8 but the emission from the fluorescent tube (cold cathode tube) in the Y-direction which is perpendicular to the X-direction is repeatedly reflected by the reflector 52 and advances in the Z-direction (such a direction as the liquid crystal display unit is disposed) which is perpendicular to the XY plane so that the emission distribution (luminance distribution) in the Y-direction is distorted (in a zigzag pattern), thus failing to uniform the luminance distribution.
Thus, in the usual backlight type display apparatus, the emission distribution (luminance distribution) in the direction perpendicular to the longitudinal direction (X-direction) of the fluorescent tube is not uniform and a streak-like directionality (linear dark areas) is produced in the emission distribution. In order to improve in uniformity of the luminance, a diffusing film having an excellent light-diffusing property may be used. However, usually, a diffusing film is scattered an incident light isotropically, and as a scattering angle becomes larger, a scattering intensity decreases greater. Referring to the degree of decrease in the scattering intensity when the scattering angle becomes larger, in the case of a commonly used diffusing film, in about 9° of the half width, the decay (extinction) of the intensity is, for example, F(0°)/F(18°)=about 12 and F(0°)/F(23°)=about 60, wherein θ represents the scattering angle, and F represents the scattered light intensity (or the intensity of scattered light), and the decay of the scattering intensity is extreme depending on the angle. Therefore, the intensity of light scattered at 30° or more of the scattering angle is very small.
From such a viewpoint, use of a prism sheet improves the luminance at a scattering angle from the front to 20°. That is, regarding the direction that a prism sheet focuses a light, use of one piece of the prism sheet can make widely the scattering angle to about 18°. However, if the scattering angle is more than 18°, the scattering intensity (luminance) decreases rapidly. In case of the method in which two pieces of the prism sheet is disposed in the direction perpendicular to each other, the luminance of the display apparatus can be uniform isotropically without depending on the angle. However, the angle is not more than about 20° in length and breadth. When the scattering angle is more than 20°, the luminance is deteriorated rapidly in comparison with the luminance in the case of using no prism sheets.
Such display apparatus restricts an angle of vision (visual angle or viewing angle) for a user of the display apparatus, and therefore, the user can not recognize the display of the display surface visually in a wide angle. Thus the display apparatus is inconvenient and effects a sense of fatigue. Therefore, a diffusing sheet, which can scatter a light over a wide range of the angle has investigated. However, in such a diffusing sheet, the luminance is extremely deteriorated. Accordingly, in order to improve in the luminance, a light source having strong emission ability should be used.
Japanese Patent Application Laid-Open No. 314522/1992 (JP-4-314522A) describes an anisotropic light-scattering material comprising a transparent matrix and a transparent substance which is morphologically anisotropic and differing in the index of refraction (the refraction index) from the transparent matrix as uniformly dispersed in the matrix in a positional relation shifted in an orderly and mutually parallel manner. Moreover, the literature discloses the preferred range of the aspect ratio of morphologically anisotropic substance is 15 to 30 and the length of minor axis is 1 to 2 μm. Specifically, the anisotropic light-scattering material is manufactured by a method which comprises kneading a low-melting low-density polyethylene for the transparent matrix resin with a high-melting polystyrene or a styrene-acrylonitrile copolymer for the transparent substance, extruding the resulting composition, and cooling the molten resin extruded in the form of a sheet under stretching with a large draft in the direction of extrusion. The anisotropic light-scattering material has been used as a lenticular lens for the projection television screen.
Japanese Patent Application Laid-Open No. 114013/1995 (JP-7-114013A) discloses a liquid crystal display apparatus in which a film or a sheet capable of scattering and transmitting an incident light is disposed on a display screen in order to improve in viewing angle properties. The literature discloses a film or a sheet in which a dispersed phase particle composed of a transparent resin and having a ratio of longitudinal axis to minor axis of not less than 10 and an average particle size of 0.5 to 70 μm is dispersed in a transparent resin matrix.
However, in a display apparatus with the use of a tubular light source having anisotropy in an emission distribution (luminance distribution), it is difficult to illuminate a display panel with uniform luminance even if using these films or sheets.
It is, therefore, an object of the present invention to provide an anisotropic scattering or diffusing sheet which is capable of inhibiting the decrease or deterioration of the luminance depending on an angle relative to a display surface of a transmittable display apparatus (in particular, a transmittable liquid crystal display apparatus) and capable of decreasing an angle dependence on the luminance, a plane or flat light source unit utilizing the sheet, and a transmittable display apparatus utilizing the plane or flat light source unit (a transmittable liquid crystal display apparatus).
It is another object of the present invention to provide an anisotropic scattering or diffusing sheet which is capable of expanding or enlarging a visual angle relative to a display surface and recognizing the display surface visually in the high luminance, a plane or flat light source unit utilizing the sheet, and a transmittable display apparatus utilizing the sheet (a transmittable liquid crystal display apparatus).
It is still another object of the present invention to provide an anisotropic scattering or diffusing sheet which is capable of suppressing the deterioration of the luminance in a certain direction even when an angle relative to a display surface exceeds 20°, a plane or flat light source unit utilizing the sheet, and a transmittable display apparatus utilizing the plane or flat light source unit.
It is further object of the present invention to provide an anisotropic scattering or diffusing sheet which is capable of inhibiting the deterioration of the luminance in spite of using a tubular light source which has an anisotropic emission distribution (luminance distribution) and is useful for illuminating a display panel uniformly, a plane or flat light source unit utilizing the sheet, and a display apparatus comprising the unit (in particular, a liquid crystal display apparatus).
It is still further object of the present invention to provide an anisotropic scattering or diffusing sheet which is capable of simplifying the structure, moreover illuminating a display panel uniformly and visually recognizing a display data clearly or finely, a plane or flat light source unit utilizing the sheet, and a display apparatus utilizing the unit (in particular, a liquid crystal display apparatus).